1. Field of the Invention
The present invention relates to a steel for carburized gear capable of realizing high fatigue strength and long endurance life by the conventional heat treatment comprising the steps of gas carburization, quenching and tempering. The industrial applications includes the automobile, construction vehicle and industrial machine industries wherein gears are widely used.
2. Description of the Prior Art
For improving the fatigue strength and endurance life of gears treated by gas carburization, quenching and tempering, various techniques have been proposed including the one disclosed in Japanese Patent Application Laid-Open No. 83848/1922. The amounts of Si, M, Cr and the like which are oxidized more easily than Fe are reduced in the steel in order to reduce the interranular oxidation or incompletely hardened layer which cause fatigue cracks, while the hardenability ad mechanical properties thereof are regulated by the incorporation of Ni, Mo or the like which have a resistance to oxidation greater than that of Fe. Such techniques also include one in which a fine spherical carbide is precipitated in the surface part of the steel so as to increase the hardness of the surface by enhancing the carbon potential at the time of carburization. This technique is being generally known as high-concentration carburization, plasma carburization or excess carburization. The various techniques further include one in which a residual surface compression stress is imparted to the steel by shot peening so as to retard the progress of fatigue cracks.
However, all the above techniques for improvements are concerned with the properties of gears prior to actual use, and do not contemplate the gears in actual use, namely in a gearing state under imposed load. Especially, when the driving and driven faces of gears contact each other at a high contact surface pressure, a surface fatigue phenomenon arises which cannot be dealt with only by the contemplation of the properties of the gears prior to use. Additionally, in the recent failures of gears, contact surface fatigue is most predominant in accordance with the demands for higher engine output and promotion of gear miniaturization.
More specifically, in the actual use and gearing state of the gears, it is conceivable that the temperature of the contact surface of the gears is raised to 200.degree.-300.degree. C. by the friction under contact surface pressure inclusive of slip. When exposed to such high temperatures the hardness of the carburized case is decreased as compared with that prior to use.
Maintaining the hardness of the carburized case is the most important factor combatting the surface fatigue. There has been an unsolved problem that, even if the hardness of the carburized case prior to use is improved by the above techniques for improvements, the decrease of the hardness of the carburized case attributed to the frictional heat during use brings about surface fatigue.
In order to solve the above problem easily at a low cost, the present invention has developed a steel for carburized gear capable of providing the gear with softening resistance through the conventional steps of gas carburization, quenching and tempering without resort to any special heat treatment, by regulating the chemical composition of a steel as a material to be carburized. The gist of the present invention resides in utilizing Si which is an element having effective softening resistance. It is believed that Si acts to retard the diffusion of carbon owing to the chemical repulsive force thereof to C to thereby inhibit the formation and cohesion of a carbide which is the cause of the softening of the steel. However, Si is a strong ferrite stabilizing element, so that there is a problem that it elevates the .gamma..fwdarw..alpha. a phase transformation initiating temperature of the steel to thereby induce a ferrite formation in a core part structure having a less carbon content at the customary quenching temperature after carburization. The formation of a ferrite is detrimental to strength because it renders the microstructure of the steel nonuniform and thereby preferentially advance cracks. A further problem is that Si is an element in the presence of which an intergranular oxidation is very likely to occur at the time of carburization.